The invention relates to a linear compressor or a refrigerating unit comprising a piston housing and a compressor piston movable back and forth therein along an axis, whereby the compressor piston is mounted in the piston housing by means of a housing wall having openings and a gaseous fluid flowing through the openings.
In oil-free linear compressors a compressor piston is separated from the housing wall by a cushion of gaseous refrigerant which flows into the compressor piston through micro-openings through a housing wall of a piston housing. To maintain this gas pressure bearing provided by the cushion it is necessary to have a continuous inflow of gas as otherwise contact takes place between the compressor piston and the housing wall, causing friction and therefore wear. It is a known approach to form the gas cushion by means of numerous micro-holes drilled in the cylinder wall. U.S. Pat. No. 6,575,716 provides for a circumferential groove in the housing wall with a central supply hole.
During the startup phase of the compressor, which usually takes several minutes until the compressor reaches its working temperature, a partial amount of the refrigerant compressed by the compressor may condense owing to a low temperature accompanied by high pressure. The condensate mainly forms on the outside of the housing wall designed as a cylinder sleeve, which wets and blocks the micro-holes drilled in the housing wall. This wetting of the micro-nozzles considerably impedes the inflow of gas needed for the gas pressure bearing and, if large areas are wetted, can lead to inadequate functioning of the gas pressure bearing. This condensation effect can be exacerbated by the pressure difference in front of and behind the micro-hole if a refrigerant evaporates on the inner wall of the housing as such evaporation causes the housing wall to become colder.
The condition of the micro-holes being blocked by refrigerant condensation usually lasts for about ten minutes. It can, however, last for much longer. It only ends when the friction of the compressor piston on the housing wall and the compression heat have heated up the entire system adequately for a critical temperature range to have been exceeded.
Under certain circumstances, the evaporation coldness can stabilize the condensation of the refrigerant, so that the frictional heat is not enough to take the temperature above the critical range and only when considerable damage is caused to the linear compressor is the friction high enough to produce enough heat. This is an undesirable situation, however, as it reduces the efficiency of the linear compressor and shortens its service life.
Particularly hard surface coatings are applied to the compressor piston to reduce the wear caused by the frictional phases during startup and slowdown to an acceptable level. Such surface coatings are, however, comparatively expensive.
A suitable heat bridge between the pressure side of the linear compressor and the gas pressure bearing can be used to prevent ongoing condensation, but this entails a loss in performance during the startup phase.